Electric Power Principles, 2nd Edition

3Transmission Lines

Transmission lines are open‐air wires or insulated cables that carry electric power from one place to another. In the context of this book, “transmission lines” are taken to mean not only the very high voltage lines that are referred to as “transmission” by utility engineers, but also lower (but still high) voltage lines referred to as “subtransmission” and “distribution.” They all carry electric power and they all behave in roughly the same way. Transmission lines operate at high voltage to reduce losses because loss is proportional to the square of current, and for a given amount of power carried, current is inversely proportional to voltage.

Transmission lines, being current‐carrying wires, produce magnetic fields and therefore exhibit inductance that can be described as being in series with the terminals of the line. Since they carry high voltage, they also produce electric fields that are terminated on the conductors, so that transmission lines also exhibit capacitance that is generally described as being in shunt with the terminals of the line. So a model of a transmission line that does a decent job of describing the behavior of the line at “low” frequencies is as shown in Figure 3.1. This is often referred to as a “pi” model for the line because of its form. Note that the series resistance of the line is neglected in this model. This is a limitation on the accuracy of line representation, but is usually justified by the notion (if not fact) that ...